Variadics

Variadic generics in stable Rust

Variadic Generics?

Variadic generics are one of the most discussed potential Rust features. They would enable traits, functions, and data structures to be generic over variable length tuples of arbitrary types.

Currently you can only implement generic code for tuples of a specific length. If you want to handle tuples of varying lengths you must write a separate implementation for each length. This leads to the notorious limitation that traits in Rust generally only apply for tuples up to length 12.

Variadic generics allow generic code to handle tuples of any length.

Tuple lists

Although variadic generics fundamentally require changing the Rust compiler, we can emulate pretty well with tuple lists.

Any tuple (A, B, C, D) can be mapped to (and from) a recursive tuple (A, (B, (C, (D, ())))).

Each element consists of a nested pair (Item, Rest), where Item is tuple element and Rest is the rest of the list. For last element Rest is a unit tuple (). Unlike regular flat tuples, these recursive tuples can be effectively reasoned about in stable Rust.

You may recognize this fundamental structure from cons lists in Lisp as well as HLists in Haskell.

This crate calls these lists "variadics" and provides traits and macros to allow simple, ergonimc use of them.

Usage

var_expr! creates variadic values/expressions, var_type! creates variadic types, and var_args! creates variadic patterns (used in unpacking arguments, on the left side of let declarations, etc.).

These macros support the "spread" syntax ..., also known as "splat". For example, var_expr!(a, ...var_b, ...var_c, d) will concatenate a, the items of var_b, the items of var_c and d together into a single variadic list.

Acknowledgements

This crate is based on tuple_list by VFLashM, which is MIT licensed:

Copyright (c) 2020 Valerii Lashmanov

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